Different types of dental plaque predispose to either caries or periodontal disease. Bacteria are intimately involved in both processes. Their role may be simply over-growth in areas where hygiene cannot be maintained or certain bacteria may be etiologically responsible. The present proposal will examine both hypotheses as plaque DNA, bacterial number and bacterial types will be determined. Pooling of plaque samples will be avoided so that the information obtained can be correlated with specific site which at the time of sampling exhibited disease or no disease. The amounts of plaque that will be removed are too small to be accurately weighed and will instead be quantitated by measurement of deoxyribonucleic acid. The plaques will be screened for the presence of Streptococcus mutans, Streptococcus sanguis and various Actinomyces species. A significant association between cavitation and Strep. mutans has been found. An inverse relationship between Strep. mutans and Strep. sanguis also exists. Simple diagnostic broths which exploit this inverse relation between Strep. mutans and Strep. sanguis are being developed and tested in clinic situations. Small samples of plaque are also being examined for acid production and metabolism of sucrose under resting cell conditions. These latter procedures require the use and/or development of micro-methods.